The present invention relates generally to tunable and re-configurable microwave systems, and, in particular, to the fabrication of re-configurable silicon-based integrated circuits, with integrated electromechanical switches and capacitors.
Microelectromechanical switches (MEMS) have been shown to have very low losses at very high frequencies. Compared to traditional active microwave switches based on transistors or diodes, the quality factors (i.e., 1/Ron Coff, where Ron is the resistance of the switch in the ON-state and Coff is the capacitance in the OFF-state) of MEMS switches are very high. Therefore, MEMS microwave components are suitable for in many types of applications.
High quality MEMS switches enable the construction of electrical systems with greatly improved functionality and flexibility. Such systems can be electrically re-configured to perform many different electrical functions without a loss of significant operating quality. However, if the electromechanical switches, control circuitry for the switches, and conductive traces among which the electrical reconfiguration is done are fabricated on different substrates, such benefits would not be as significant. Monolithic fabrication is very important for achieving the quality, reliability, functionality, and low-cost of such MEMS systems.
The present invention is directed to a low-loss micro-electromechanical microwave switch and a micro-electromechanical tunable capacitor monolithically integrated with low-cost silicon-based integrated circuits. The microwave switch of the present invention is capable of handling signals from DC to millimeter-wave frequencies. Both the switch and tunable capacitor include movable beams actuated either by thermo-mechanical or electrostatic forces. The movable beams are formed by selectively removing the supporting silicon underneath the thin films available in a silicon-based integrated circuit technology, which incorporates at least one polysilicon layer and two metallization layers. A cavity and a thick, low-loss metallization layer are used to form an electrode above the movable beam. A thick mechanical support layer is formed in regions where the cavity is located, or the substrate is bulk-micromachined (i.e., etched).
The devices are fabricated directly on finished silicon-based integrated circuit wafers, such as CMOS, BiCMOS or bipolar wafers. The present invention uses monolithic integration wherein the MEMS devices are connected to the integrated circuits necessary to control their operation, the integrated circuits being on the same substrate as the MEMS devices they control. In the present invention, this processing is performed on non-active circuit areas, i.e., where xe2x80x9cpassivexe2x80x9d components, such as resistors, capacitors, inductors, interconnections, etc. are located. The functions and operation characteristics of active circuits do not change as a results of the process sequence of the present invention.
Reconfiguration capability is an advantage of the MEMS-IC integration of the present invention. For example, a frequency selective filter based on MEMS devices, such as MEMS switches and MEMS tunable capacitors and/or inductors allows the switches to switch-in (or out) selected passive component(s) to the circuit configuration of the filter. By switching in and out electrical components into the circuit configuration, the overall circuit can be changed. Thus, for example, a passive LC filter can be changed from low-pass filter to bandpass filter by switching-in a selected set of inductors and capacitors.